This invention relates to photoluminescent articles made from electron trapping optical material and a process for making such articles.
Materials that contain luminescent centers often include one or more types of centers that trap electrons. Upon application of suitable wavelengths of light, or application of x-rays, or other radiation, the materials produce free electrons which may be trapped in an energy level higher than their ground state. If the "depth" of the trap (that is, the amount of energy required to release the electron from the trap) is large relative to the thermal energy of the ambient temperature, the electron will remain trapped for a long time. Indeed, if the trap is sufficiently deep, the electron will remain trapped almost indefinitely unless the electron is energized by energy from light, other electromagnetic energy, or thermal energy much higher than room temperature.
As used herein, a "photoluminescent material" is a material wherein electrons trapped at high energy levels due to application of optical energy will remain trapped until light or other radiation is applied to provide sufficient energy to the electron to escape from the trap. For such photoluminescent materials, room temperature thermal energy is insufficient to allow any significant portion of trapped electrons to escape from their traps. As used herein, "optical energy" shall include visible light, infrared light, and ultraviolet light unless otherwise noted.
Although various photoluminescent materials have heretofore been known, the properties of such materials have often been less than desirable. For example, photoluminescent materials have been used for locating infrared beams by outputing visible light upon placement of the material within an infrared beam, but such previous photoluminescent materials are not sensitive enough to detect relatively low levels of infrared light, The visible light output by such materials is often at a quite low level such that detection of the visible light is difficult.
Further, such materials commonly have insufficient depth for the electron traps and/or a relatively low density of electron traps such that it is difficult to maintain the electrons trapped for extended periods of time, The ratio of the energy of light input, in order to trap electrons, to energy of light output, by the freeing of the trapped electrons, in such materials in order to trap electrons by the freeing of the trapped electrons is often quite high. That is, a relatively large amount of energy must be put into the material to provide a given output optical energy. The development of photoluminescent materials avoiding or minimizing the disadvantages discussed above would open up numerous other applications for such materials.
Such photoluminescent materials have been contemplated for use as storage media for optical memory systems; however, conventional methods of forming a layer of photoluminescent material on a substrate for optical memory applications have produced a relatively thick layer of material. That thickness inherently promotes dispersion of the write and read beams of light, as well as the dispersion of the escape of electrons. Also, that thickness inherently contributes to a lower response time than is desirable.
Another limitation of prior methods is that they frequently employ a heating step in which the substrate is heated to relatively high temperatures. This limits the types of substrates that may be employed, as low-melting-point substrates tend to deform at such temperatures or react with the photoluminescent material.
A further limitation of certain potoluminescent materials is that they contain lithium fluoride to promote fusability, the fluoride ion in which tends to react with glass substrates.